Electrowinning of metals



United States Patent 'Ofiice 2,798,040 Patented July 2, 1957 ELECTROWINNING OF METALS David J. Pye, Concord, and George F. Schurz, Oakley,

Califi, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application September 15, 1955, Serial No. 534,596

8 Claims. (Cl. 204-108) This invention is concerned with the electrowinning of metals and is particularly directed to an improved process and to a novel cell feed composition for electrowinning operations.

In recent years increasing amounts of metals, particularly copper and zinc, have been recovered by the leaching of ore to extract the metal values in the form of soluble salts in solution. Thereafter such solution is submitted to electrolysis to separate the metal. Such electrowinning processes have encountered certain difiiculties. For example, the deposit of metal on .the cathode is frequently non-uniform and tends to build up irregular trees which may grow across the space between the cathode and anode causing a short circuit in the cell. Further, such non-uniform deposits frequently show poor adhesion to the cathode so that portions of the deposit break off and redissolve with a resultant loss in current efficiency.

It is an object of the present invention to provide an improved method for the electrowinning of metals. It is a further object to provide a method for the electrowinning of metals whereby uniformly adherent deposits of metal are obtained. Another object is to provide a method for minimizing the growth of trees in electrolytic deposits of metal. An additional object is to provide an improved and novel cell feed composition. Other objects will become apparent from the following specification and claims.

According to the present invention, it has been discovered that the incorporation of small amounts of acrylamide polymers in the electrolytic cell feed employed in the electrowinning of metals from aqueous solution results in improved current efiiciencies and in the formation of a smooth, dense, uniform deposit of metal.

The acrylamide polymers employed in accordance with the present invention are Water soluble and characterized by a minimum of cross linking between polymer chains. Effective polymers are those having viscosities of at least about 2 centipoises for an aqueous 0.5 percent by Weight solution thereof in distilled water adjusted to a pH of 3 to 3.5 and at a temperature of 25 C. as determined with an Ostwald viscosimeter. The term viscosity, as employed in the following specification and claims, refers to the viscosity of an aqueous 0.5 percent by Weight solution under the conditions set forth above. The term acrylamide polymer includes both the homopolymer of acrylamide and copolymers of acrylamide with up to about 25 mole percent of other suitable monomers such as acrylic and methacrylic acids and their lower alkyl esters, acrylonitrile, methacrylonitrile, methacrylamide, vinyl alkyl ethers, styrene, vinyl chloride, vinylidene chloride and the like, each such copolymer being characterized by Water solubility, low degree of cross linking and viscosity properties as set forth above.

In carrying out the invention, the acrylamide polymer is added to the cell feed in any suitable fashion, provided that the uniform distribution of the polymer in dissolved form in the cell feed is accomplished. Thus, for example, solid acrylamide polymer may be added in finely powdered form to the cell feed with vigorous agitation to accomplish the solution and distribution of the polymer. In a preferred mode of operation, the acrylamide polymer is dissolved in Water or in a portion of the untreated cell feed to produce a concentrate solution and the latter is added to the cell feed in any suitable fashion. In general, it is desirable to meter the concentrate solution into the cell feed under conditions such as to provide for thorough mixing of the polymer with the feed before introducing the mixture into the electrolyticcells. However, if necessary, as for example, in adjusting the con centration of acrylamide polymer in the cell feed composition, a solution of the polymer may be added directly to the electrolytic cell while electrolysis is proceeding. Following the addition of the acrylamide polymer, the resulting cell feed composition is electrolyzed in conventional fashion to accomplish the deposition of the desired metal.

The cell feeds employed in the operations, as set forth above, are, in general, those obtained in conventional hydrometallurgical leaching operations followed by purification steps as required for the particular metal. Cell feeds for the electrowinning of zinc usually contain from about to about 220 grams of zinc, chiefly in the form of zinc sulfate, per liter of cell feed. Such solutions are substantially free of iron and copper. For the electrowinning of copper, conventional cell feeds contain from about 20 to 70 grams of copper per liter of cell feed. In such operation, the cell feed electrolyte usually contains a substantial proportion of sulfuric acid and is essentially free of chlorides.

The amount of acrylamide polymer employed in the cell feed compositions will vary depending upon such factors as the particular metal being recovered and the type and concentration of impurities in the electrolyte. In general, the acrylamide polymer is employed in amount sufiicient to accomplish improved deposition of the metal concerned. In most instances, from about 25 to about parts by weight of acrylamide polymer is used for each million parts by Weight of cell feed. The concentration of acrylamide polymer in the concentrate solution added to the cell feed is not critical provided such solution is sutficiently fluid to allow for ease of mixing with the cell feed. Thus, for example, with polymers having viscosities of from about 2 to about 5 centipoises, treating solutions may contain up to about 2 percent by Weight of polymer. With polymers having higher viscosities, it is generally convenient to employ treating solutions containing not more than about 1 percent by weight of polymer.

In a representative operation, an electrolyte solution This electrolyte solution was representative of solutions obtained by sulfuric acid leaching of oxidized zinc ore concentrates as employed for electrowinning operations. A portion of the above solution was mixed with an aqueous 0.8 percent by weight solution of an acrylamide polymer characterized by a viscosity of 9.8 centipoises. The

v 3 solutions were mixed in the proportions of 1000 parts by Weight of electrolyte solution to 4.8 parts by weight of polymer solution to provide a cell feed composition containing 38 parts by weight of acrylamide polymer per million parts of solution. This cell feed compositionwas placed in an electrolytic cell equipped with a stirrer and analuminum cathode and lead anode. A portion of the unmodified electrolyte solution was placed in an exactly similar cell and current was passed through the two :cells in seriesfor a period of three hours to accomplish electro-deposition of Zinc on the cathode. The current density was about 43 amperes per square foot of-electrode surface and the voltage drop was about 5 volts per cell. 0n completion of the electrolysis, the cathodes were removed from the cells, washed thoroughly with distilled water,

dried and weighed. It was found that the cathode from the cell containing the unmodified electrolyte solution had a rough, dark, porous deposit with many trees growing from the surface and the current efficiency for this cell was 23 percent. The cathode deposit from the cell containing the electrolyte modified with acrylamide polymer was smooth, dense and bright with very little tree formation and the current efficiency for this cell was 58 percent.

In further determinations, similar to the above, em-

ploying amounts of acrylamide polymer varying from 30 to 150 parts by weight per million parts by weight of electrolyte solution, the voltage drop per cell was varied over a range of from about 3.5 to 6.5 volts and the deposition time increased to 5, 13 and 16 hours. In all cases the current efiiciencies for the cell feed compositions containing acrylarnide polymer were increased over current efficiencies for unmodified cell feeds electrolyzed in series with therespective modified compositions. The cathode deposits of zinc from the modified cell feed compositions were uniformly smoother, denser and brighter than the deposits from the corresponding unmodified cell feeds.

By operating in similar fashion with cell feed compositions containing from about 20 to 70 grams of copper per liter in the form of copper sulfate and from about 20 to 100 parts by weight of acrylamide polymer per million parts of composition and electrolyzing at a current density of about 16 amperes per square foot, uniform, smooth, bright cathode deposits of copper are obtained.

We claim:

1. A process for the electrowinning of a metal of the group consisting of copper and zinc which comprises electrolyzing an aqueous solution of the sulfate of said metal, said solution containing in dissolved form from about 25 to about 150 parts by weight of a water-soluble acrylamide polymer per million parts by weight of solution, said polymer being selected from the group consisting of acrylamide homopolymers and acrylamide copolymers containing in combined form at least 75 mole percent of acrylamide.

2. A process according to claim 1, wherein the acrylamide polymer is characterized by a viscosity of at least about 2 centipoises for an aqueous 0.5 percent by weight solution thereof in distilled water adjusted to a pH of 3 to 3.5 and at a temperature of 25 C. as determined with an Ostwald viscosimeter.

3. A process for the electrowinning of copper which comprises electrolyzing a copper sulfate solution containing in dissolved form from about 25 to about 150 parts by weight of a water-soluble acrylarnide polymer per milselected from the group consisting of acrylamide homepolymers and acrylamide copolymers containing in combined form at least 75 mole percent of acrylamide.

5. A cell feed composition which comprises an aqueoussolution of a'metal salt selected from the group consisting of copper sulfate and zinc sulfate, adapted to be employed for the electrowinning of the metal therefrom, and incorporated'therein a water-soluble acrylarnide polymer selected from the group consisting of acrylamide homopolymers and acrylamide copolymers containing in combined form at least 75 mole percent of acrylamide, said polymer being incorporated in amount suflicient to accomplish improved deposition of the metal upon electrolysis of the feed composition.

6. A composition in accordance with claim 5' wherein the acrylamide polymer is characterized by a viscosity of at least. about 2 centipoises for an aqueous 0.5 percent by weight solution thereof in distilled water adjusted to a pH of 3 to 3.5 and at a; temperature of 25 C. as determined with an Ostwald viscosimeter.

7. A cell feed composition for the electrowinning of zinc which comprises an aqueous solution containing from about 100 to about 200 grams of zinc, chiefly as zinc sulfate, per liter of solution and from about 25 to about 150 parts by weight of an acrylamide polymer per million parts by weight of solution, said polymer being selected from the group consisting of acrylamide homopolymers and acrylamide copolymers containing in combined form at least 75 mole percent of acrylamide.

8. A cell feed composition for the electrowinning of copper which comprises'an acidic aqueous solution containing from about 20 to about grams of copper in the form of copper sulfate per liter of solution, and from about 20 to 100 parts by weight of a water-soluble acrylamide polymer per million parts by weight of solution, said polymer being selected from the group consisting of acrylamide homopolymers and acrylamide copolymers containing in combined form at least mole percent of acrylamide.

e No references cited. 

1. A PROCESS FOR THE ELECTROWINNING OF A METAL OF THEE GROUP CONSISTING OF COPPER AND ZINC WHICH COMPRISES ELECTROLYZING AN AQUEOUS SOLUTION OF THE SULFATE OF SAID METAL, SAID SOLUTION CONTAINING IN DISSOLVED FORM FROM ABOUT 25 TO ABOUT 150 PARTS BY WEIGHT OF A WATER-SOLUBLE ACRYLAMIDE POLYMER PER MILLION PARTS BY WEIGHT OF SOLUTION, SAID POLYMER BEING SELECTED FROM THE GROUP CONSISTING OF ACRYLAMIDE HOMOPOLYMERS AND ACRYLAMIDE COPOLYMERS CONTAINING IN COMBINED FORM AT LEAST 75 MOLE PERCENT OF ACRYLAMIDE. 